The main issue (besides Ambrose being fucking awesome in this role) is mic work. Rollins is good (better than most expected), but still not a guy that's going to be able to carry a faction like this, Reigns has improved immensely, but at this point he's average at best, and Ohno's probably on Rollins' level. A lot of the reason the Shield took off is those awesome promo's and backstage terrorist-y cam videos. They were and are top quality work, all carried by Ambrose. A group like this needs a charismatic leader to spread the propaganda, and they NEED Ambrose for that reason. The group is no where near as good without Rollins or Reigns. The group doesn't last without Ambrose.

They picked the perfect three guys for the group. They all bring something different and all compliment each other beautifully. The cover every area of the talent spectrum in wrestling, and I don't care if you picked the three most talented wrestlers on the roster, the Shield is not as successful with any other combination of people that isn't Ambrose, Rollins, and Reigns. They bring so many intangibles as a unit that can't be duplicated. That's why I'm so against another member. It would throw off the entire synergy of a group. You hit a home run choosing these three, why fix what isn't broke?

The Shield is Ambrose's launch pad, as well as Rollins and Reigns. None of these guys are thirty years old yet and arguably have all put themselves on the map already with solid matches and promos respectively.

This is just the beginning, they aren't going to let Ambrose loose when he hasn't even been on the main roster for a full year yet. Why would you let everyone see the full potential/what a talent has to offer when this guy is most likely going to be one of your top guys further down the line. You are shooting yourself in the foot business wise if you do so, really.

As for Ohno, he would've been able to hold his own in the ring without a doubt, but as for the promos - I really don't see it. To have both him and Reigns in the faction would've been almost too many weaknesses to cover, regardless of his main weakness only being his look. With Ambrose solid on the mic and Rollins solid in the ring they have the two grounds covered to make their "weaker" member in Reigns able to learn both elements as he went along, whilst Rollins/Ambrose bettered their craft too regarding working live TV and weren't considered laughing stocks by being part of a faction that didn't have a menacing looking guy.

Obviously, the main basis of the three guys goes:

Rollins - Ring Worker
Reigns - Muscle
Ambrose - Talker

Sure they can all work a ring and a mic, maybe Reigns for me is the only one who needs to better the way he delivers his words but that's about it and from what I've seen he has improved. Therefore 'The Shield' has done its job, whilst getting three talents that deserved TV time out there for viewers to see.

The Shield would never have gotten over nearly to the extent they have if Ohno had filled Ambrose's role.

Much of the spark of their early videos and segments came from Ambrose's incredible work on the mic - Rollins and Reigns were in supporting roles. If you replace Ambrose with a guy like Ohno, who's a great worker but has the mic skills of an empty cardboard box, you take away one of the most successful and engaging aspects of the group. No thanks.